(1) Field of the Invention
The present invention relates to a reflow soldering apparatus for soldering electronic members to a circuit substrate with a hot gas (which is obtained by heating) of various gases, especially, to the economical reflow soldering apparatus having a small temperature fluctuation at a pre-heating chamber and a reflow chamber for main heating, an ability to maintain a desired temperature profile, and a preferable solderability.
(2) Description of the Prior Art
Currently, technology for high density packaging and surface mounting of a substrate have progressed, and a reflow soldering method is widely adopted for soldering electronic members to a circuit substrate in view of reliability and productivity of the soldering. Furthermore, solder particles in soldering paste are becoming finer than ever and solid substances in soldering flux are becoming less than ever.
With the soldering paste changing in accordance with the above described tendency, conventional soldering of the circuit substrate by the reflow soldering method with hot air frequently causes solder balls and a lack of wettability on account of oxidation of the solder particles and insufficiency of an activating agent.
Therefore, the reflow soldering method using an inert hot gas such as nitrogen instead of air has become widely adopted. For example, JP-A-64-71571 (1989) discloses a representative reflow apparatus for the above described reflow soldering method. FIG. 8 is a schematic vertical cross section illustrating a composition of the conventional reflow soldering apparatus which is disclosed in the above example, JP-A-64-71571 (1989).
Referring to FIG. 8, the reflow soldering apparatus comprises three units for a preheating zone (a preheating chamber), two units for a main heating zone (a reflow chamber), and two units for a cooling zone (a cooling chamber).
As for a heater, a gas blow type infrared heater 54 is adopted for all of the chambers, and a pair of the heaters is provided at above and beneath a processing objects conveyer (not shown), respectively. Nozzles 55 which are adjustable in a gas blow out direction are provided respectively among each of the infrared heaters 54 in a manner making a pair by above and beneath the conveyer. The gas for heating is supplied to each of the infrared heaters 54 through a line 60 as a blow gas, and the gas from a line 61 is supplied to the nozzles 55 provided among each of the infrared heaters 54 and, subsequently, is blown into the chambers. The gas blown from among each of the infrared heaters 54 is recovered through a line 62, and is supplied to a cooling nozzle 56 through a blower 58 and a heat exchanger 59.
The processing objects (not shown) are transferred by a conveyer (not shown in the figure) into the apparatus from right side in FIG. 8, preheated to a designated temperature by three pairs of the infrared heaters 54 at the preheating zone, further heated to a temperature for melting the solder by two pairs of the infrared heaters 54 at the main heating zone, and subsequently cooled by cooling nozzles 56 and cooling fans 57 at the cooling zone to finish the soldering process.
The above described prior art has the problems:
(i) Flow velocity of the hot gas which contributes to a convective heat transfer is restricted by a quantity of the hot gas supply because a blower for circulating the hot gas is not provided. On the other hand, it is necessary to minimize a quantity of gas supply (a quantity of consuming gas) because of expensiveness of the hot gas, and consequently, a radiation heat transfer by infrared becomes rather a main heat source than the convection heat transfer. Therefore, the reliability of the soldering decreases on account of local heating of the processing objects caused by each different colors and each different heat capacities of the members from others; PA1 (ii) Flow directions of almost of the hot gas become horizontal when the hot gas is impinged upon the processing objects perpendicularly from above and beneath the processing objects. However, flow velocity of the hot gas in an inlet and outlet direction of the chambers varies depending upon the electronic members on the processing objects. Accordingly, although the hot gas flows stably in a condition without the processing objects, when the processing objects are placed in the chamber, the above described balanced condition without the processing objects is interrupted, and entering of an atmosphere at a room temperature into the chamber or releasing of the hot gas from the chamber occurs in some cases. Consequently, temperature of the hot gas fluctuates, and the reliability of the soldering is decreased. Furthermore, it becomes necessary to increase the hot gas supply and to strengthen the heating for keeping the desired temperature profile in the chamber, and accordingly, economy of the process decreases; and PA1 (iii) When the balance of the hot gas flow in the inlet and outlet direction is disturbed by placement of the processing objects, quantities of hot gas leakage among each of the preheating chamber, the reflow chamber, and the cooling chamber increase, and maintaining of the desired temperature profile in each of the chambers becomes difficult. Consequently, a necessary temperature profile for soldering the circuit substrate can not be maintained and the circuit substrate which can be soldered by the apparatus is restricted, and economy of the apparatus decreases.
A reflow soldering apparatus wherein soldering is performed by impinging a hot gas to a processing object by using a blower is disclosed in JP-A-63-278668 (1988).In accordance with the last mentioned prior art, blowers are installed at both an upper side and a lower side of a conveyer in a staggered manner, respectively, and a combination of the blowers installed in the above described manner causes formation of a circulating stream of the hot gas. However, the hot gas is impinged to the processing object after being blown out from the blowers and heated up on a desired temperature by a heater, and a turbulence of the circulating stream caused by the processing object which is transferred into the circulating stream by the conveyer is not considered sufficient, hence, the hot gas releases to a next chamber and there is a problem arises in that a preferable temperature profile for heating and cooling can not be obtained.